1. Field of the Invention
The present invention relates to a garnet film for magnetic bubbles, which is suitable as a magnetic bubble holding film in a magnetic bubble memory element.
2. Description of the Prior Art
As is well known in the art, a magnetic bubble memory element has attracted the attention as a promising information processing element, especially, as a memory element and has been actively developed. In case the magnetic bubble memory element is used as the memory element, it is the diameter (d) of the magnetic bubbles that determines the memory density which is the most important factor of the function of the memory element.
At the present stage, magnetic bubbles having a diameter of about 3 to 5 .mu.m are generally used. If the diameter can be made equal to or smaller than 2.5 .mu.m, for example, it is possible to drastically increase the memory density.
Specifically, in order that the magnetic bubbles may be practised in the memory element in place of the other memory element such as the disc memory or a semi-conductor memory being generally used at present, their diameter has to be so remarkably reduced that the memory density may be drastically enhanced. For that requirement, a magnetic garnet film, in which the bubbles with small diameter can stably exist and operate with satisfactory operating characteristics, has to be found out. However, the magnetic garnet film for the so-called "small bubbles" having a diameter equal to or less than 2.5 .mu.m is known to generally have a large temperature change of a bubble collapse field H.sub.o.
For example, in the case of a (YSmLu).sub.3 (FeGa).sub.5 O.sub.12 film which can support bubbles having a diameter of about 2 .mu.m, the temperature coefficient of H.sub.o at 30.degree. C. is -0.30 to -0.35%/.degree.C.
On the other hand, the temperature coefficient of the bias magnetic field produced by a barium ferrite magnet, which is conventionally used as a bias magnet, is -0.20%/.degree.C. so that a considerable difference exists between H.sub.o and a bias field.
If the temperature variation of H.sub.o in the bubble film is different from the temperature variation of the aforementioned bias magnetic field, the temperature range within which the bubbles can stably operate is remarkably narrowed. It is therefore apparent that such large difference is not favorable in the magnetic bubble memory element.
On the other hand, if the diameter of bubbles becomes remarkably small, the saturation magnetic flux density 4.pi.M.sub.s becomes remarkably large thereby to make it difficult to operate the small bubbles stably at a high speed.
The following references, for example, are known as to the temperature characteristics of the garnet film for the magnetic bubble memory element:
(1) R. M. Sandfort, et al., "Temperature variation of Magnetic Bubble garnet film parameters", AIP Conf. Proc. 18, (1), p 237 to 214 (1973);
(2) G. G. Summer, et al., "Growth Reproducibility and Temperature Dependencies of the Static Properties of YSmLuCaFeGe Garnet", AIP Conf. Proc. 34, p 157 to 159 (1776); and
(3) Jerry W. Meody, et al., "Properties of Gd.sub.y Y.sub.3-y Fe.sub.5-x Ga.sub.x O.sub.12 films grown by LPE", IEEE transactions on magnetics, Vol. May. 9, 377 (1973).
The above-identified reference (1) discloses the temperature characteristics of the garnet film for the magnetic bubble element but neither has a description relating to the improvement in the temperature characteristics of H.sub.o of the garnet film for the fine bubbles nor disclosed the composition of the present invention.
The above-identified reference (2) disclosed the YSmLuCaFeGe garnet as a material having a temperature coefficient of H.sub.o of -0.2%/.degree.C., but this is the Ca-Ge garnet which has an absolutely different composition from that of the present invention and which has its control of H.sub.o impossible so that the value of H.sub.o cannot be made the most suitable for the bias magnet used.
The above-identified reference (3) discloses a garnet containing Gd and Ga but made absolutely different from the present invention in the composition such that the ratios of Gd and Ga are different and such that Sm and Lu are not contained. Moreover, the material disclosed is not the garnet for the small bubbles, and there is no disclosure concerning H.sub.o.
On the other hand, Japanese Patent Laid-Open No. 55-62714 discloses the garnet film which has such a composition as is expressed by a general formula of (YSmLu).sub.3-x Gd.sub.x Fe.sub.5-y Ga.sub.y O.sub.12. The garnet film disclosed is common with the present invention in that Gd and Ga are contained and in that the temperature characteristics of H.sub.o are excellent but finds it very difficult to operate the remarkably small bubbles having a diameter equal to or smaller than 1 .mu.m with the satisfactory high speed operating characteristics.